The invention relates to a method and a device for controlling the power of a control channel in a radiotelephone system. In the method and device, the power of the control channel is kept constant during normal operation.
The method and the device according to the invention can be used, e.g., in the GSM mobile radio network, but it is equally applicable to radio systems of other types in which the channels are divided into separate traffic and control channels.
In the GSM radio system, the traffic channels utilize dynamic control of transmitting power both at the base station and at mobile stations. The power control word comprises four bits, which means that there are 16 power levels available. The spacing between two successive power levels is 2 dB, and so the entire power control range is 30 dB. In a so-called TX-on time slot, in which the transmission takes place, the transmitting power is adjusted in each time slot of the frame to one of the 16 different power levels in accordance with the above-mentioned power control word. A so-called TX-off time slot, in turn, is a time slot in which no transmission occurs (the RF power is zero). In this connection, EP Patent Application 414352 A2 is referred to, which describes in more detail both the GSM system and a radio transmitter belonging to it, the transmitting power of the radio transmitter being adjustable in the TX-on time slot to one of the above-mentioned 16 power levels.
One of the control channels of the GSM system is a broadcast control channel (BCCH) which is a one-way channel from the base station to a mobile station. (The control channels and the frame structure of the GSM system are described in greater detail e.g. in INSKO publication 121-89 GSM-yleiseurooppalainen matka-puhelinverkko [GSM Pan-European mobile radio network], Chapters X and XI.) The broadcast control channel transmits data concerning one specific cell (about power levels, frequencies, etc.) to the mobile station. The power control of the mobile station is based on measurements performed by the base station, and vice versa. As the measurements performed by the mobile station must have a reference level, the power transmitted at the BCCH frequency is constant in each cell. However, this power is not the same as the maximum power of the transmitter. Therefore the power of the BCCH channel should also be controllable.